Provide Windows Remote Assistance without Invite

[g33man]

I need to offer Remote Assistance to a Vista system, but without an Invite. Both machines are on the same LAN (no FW or router). If I use an invite file, it works, so I know the service is running on the Vista machine. On the machine providing the offer (Win 7), I use the Advanced Connection Option, then provide the IP of the Vista machine. I immediately get the error:

Your offer to help could not be sent
Check the following:
- Do you have the correct permissions on the remote computer?
- Is the remote computer turned on, and is it connected to the network?
- Is there a network problem?

As an alternate solution, can an Invite file be created with no expiry?

Thanks.